Electrochemical device

ABSTRACT

An electrochemical device includes an envelope having a sealable opening and a resin layer on its inner side and an electrochemical element having terminals, wherein the electrochemical element is inserted in the envelope through the opening and sealed therein. A strip of a material different from the resin layer is disposed in the envelope opening and sealed together by thermal fusion so that the strip serves as a pressure relief valve for relieving pressure within the envelope.

[0001] This invention relates to electrochemical devices such aspolymeric lithium secondary batteries and electric double-layercapacitors, and more particularly, to electrochemical devices having afailsafe mechanism against abnormal inflation and heat release.

BACKGROUND OF THE INVENTION

[0002] Various forms of batteries have been used in a wide variety ofapplications, mainly in electronic and automotive applications and asvery large size ones for power storage. In such batteries, liquidelectrolytes are often used. The replacement of liquid electrolytes bysolid ones is expected to prevent liquid leakage and enable a sheetstructure. Use of solid electrolytes is thus attractive for batteries ofthe next generation.

[0003] It is expected that if lithium ion secondary batteries which arefrequently used in cellular phones and notebook computers can befabricated to a small-size, sheet or laminate structure, theirapplication will remarkably grow. For solid electrolytes, there havebeen proposed ceramic materials, polymeric materials and compositematerials thereof. Among these, gel electrolytes in the form of polymerelectrolytes plasticized with electrolytic solutions possess both a highelectric conductivity inherent to liquid ones and a plastic characterinherent to polymeric ones and are deemed potential in the futureelectrolyte development.

[0004] One advantage of batteries using solid electrolytes is an abilityto form a thin large area structure, that is, a sheet structure. Thiswill accelerate the further development of battery applications. Theadvantages of such sheet-shaped batteries are not obtained from the useof metallic casings as used in prior art cylindrical and rectangularbatteries. Since the metallic casing accounts for a large proportion ofthe weight and thickness of the overall battery, the advantages ofsheet-shaped batteries are offset. To take advantage of sheet-shapedbatteries, it is requisite to use a lightweight laminate film as thecasing or envelope.

[0005] When any anomaly occurs in a prior art battery using such a filmas the envelope, the result is gas release and still worse, ignition,depending on the type of electrolyte. For example, a charger is designedto interrupt charging when the predetermined time or voltage is reached.If charging is not interrupted for some reason or other, the battery isover-charged in excess of its capacity. Further progress ofover-charging can cause the electrolyte to be decomposed to give offgases to inflate the envelope, eventually leading to failure of theenvelope or ignition.

[0006] Lithium ion batteries using metallic casings as the envelope arecommercialized as having explosion-proof valves built therein. In theevent of laminate film used as the envelope, it is difficult to installan explosion-proof valve and very difficult for such a valve to operateunder the necessary pressure.

[0007] One technique of providing a laminate film with a valve isdisclosed, for example, in JP-A 10-55792. The junction where opposedportions of laminate film are joined is provided with a region of alower peel strength tapered from the inside to the outside. Usually, thejunction is formed by bonding of a fusible resin. The region of lowerpeel strength is formed by introducing a non-fusible material (e.g.,nickel foil) into the fusible resin, by effecting the bonding operationat a lower temperature than in the remaining area, or by leaving theregion unbonded.

[0008] The lower peel strength region can function as a valve forventing gas when the internal pressure of the battery increases. For theregion to exert the desired function, heating conditions during thebonding and/or the non-fusible material must be appropriately selected,which is not always easy in practice. In order that the envelope oflaminate film prevent ingress of moisture and undesired contaminantsfrom the exterior, the junction must have a seal width of at least about4 mm. In the embodiment wherein the region of lower peel strength isdefined in the junction by leaving the region unbonded during thebonding step, if the narrow portion of the junction disposed outside thelower peel strength region is 4 mm, the entire junction has a seal widthfar greater than 4 mm. This is disadvantageous from the energy densitystandpoint.

[0009] To avoid such an undesired phenomenon as rupture or ignition,batteries are normally provided with protective circuits. The protectivecircuits are often designed to shut off current flow when thepredetermined voltage is reached.

[0010] However, assuming the situation where the protective circuitfails for some reason or other, a redundant protective means is oftenprovided. Typical protective means are PTC elements and heat-sensitiveprotective components such as thermal fuses. The PTC elements areelements having a positive temperature coefficient, that is, elementswhich increase their resistance in response to a temperature rise. Asharp increase of resistance occurs above a certain temperature whilethe resistance change rate becomes of three figures, and even of sixfigures for some materials.

[0011] In general, chargers are designed to conduct a constant currentflow until the predetermined voltage is reached and thereafter, controlthe current flow at the predetermined voltage. If the battery heats upby any anomaly during the charging process, the PTC element is heated toincrease its resistance, thereby restraining the charging current flowfor inhibiting further charging. The thermal fuse functions to shut offthe charging current flow when the predetermined temperature is reached,interrupting the charging process. It is well known that lithium ionsecondary batteries undergo thermal runaway above a criticaltemperature. The thermal runaway produces gases or further heat,inducing a failure or ignition of the battery. Therefore, the chargingcurrent flow must be reduced or shut off before the battery temperaturereaches the critical level.

[0012] When a protective component is attached to the surface of anenvelope, the maximum thickness of the battery including that protectivecomponent is often increased. Such a thickness increase is undesirablebecause electronic equipment such as cellular phones in which batteriesare mounted are currently required to be smaller in size and thickness.It is thus desired to attach the protective component to the battery ata location aside from the maximum thickness.

[0013] However, the surface temperature of the battery is not alwaysuniform. High heat radiating areas and low heat transfer areas havelower temperatures. Depending on the attachment location of theprotective component, there is a possibility that the protectivecomponent operates at a lower temperature than other areas. This delaysthe time when the charging operation is interrupted, failing to preventgas generation or ignition.

[0014] As mentioned above, even when the protective circuit is provided,there is a possibility that the protective circuit fails for some reasonor other. Even in such a case, safety must be insured. When only a gasventing mechanism is added, a fire hazard can still be left because thecharging operation continues even after the gas venting.

SUMMARY OF THE INVENTION

[0015] A first object of the invention is to provide an electrochemicaldevice having reliability and safety owing to a simple venting means forventing gas in response to a rise of the internal pressure, the ventingmeans serving as an explosion-proof valve and preventing ingress ofmoisture and contaminants.

[0016] A second object of the invention is to provide an electrochemicaldevice comprising an envelope of flexible film having a protectivecomponent attached thereto, which is designed, without changing themaximum thickness of the device, to ensure that heat is transferred fromthe interior to the protective component whereby the device has improvedsafety.

[0017] A third object of the invention is to provide an electrochemicaldevice, typically secondary battery, comprising an envelope of flexiblefilm and a failsafe mechanism capable of preventing current flow on agas generating accident.

[0018] In a first embodiment, the invention provides an electrochemicaldevice comprising

[0019] an envelope having a sealable opening,

[0020] an electrochemical element having terminals, the electrochemicalelement being inserted in the envelope through the opening which issealed to form a seal,

[0021] the envelope having a resin layer on its inner side adjacent theelectrochemical element, and

[0022] a strip of a material different from the resin layer of theenvelope, disposed in at least a portion of the seal, the strip servingas a means for relieving pressure within the envelope.

[0023] In a preferred embodiment, the sealable opening of the envelopeis defined by opposed portions of the resin layer of the envelope, thestrip is interposed at least in part between the opposed portions of theresin layer, and a seal is formed by joining the opposed portions of theresin layer together with the strip to provide the pressure reliefmeans.

[0024] The strip is preferably made of a resin mixture of a first resinadhesive to the resin layer of the envelope and a second resinnon-adhesive to the resin layer. Typically, the resin layer of theenvelope is made of a first polyolefin resin, and the strip is disposedin contact with the resin layer and made of a resin mixture of the firstpolyolefin resin and a second polyolefin resin non-adhesive to the firstpolyolefin resin. Specifically, either one of the first and secondpolyolefin resins comprises polypropylene, and the other resin comprisespolyethylene. Preferably, the resin mixture of which the strip is madecontains a more amount of the second polyolefin resin than the firstpolyolefin resin. Specifically, the resin mixture of which the strip ismade contains the first polyolefin resin and the second polyolefin resinin a weight ratio of from 40/60 to 15/85.

[0025] In a preferred embodiment, the terminals extend through the sealof the envelope, and the strip is disposed in the portion of the sealexcluding the location of the terminals.

[0026] In a second embodiment, the invention provides an electrochemicaldevice comprising

[0027] an envelope,

[0028] an electrochemical element received and sealed in the envelope,and

[0029] a heat-sensitive protective component for protecting theelectrochemical element, the heat-sensitive protective component beingattached to the envelope substantially outside the electrochemicalelement.

[0030] Provided that the electrochemical device has a maximum thicknessD₁ where the electrochemical element is received and a maximum thicknessD₂ where the heat-sensitive protective component lies, D₁ is equal to orgreater than D₂.

[0031] Where the electrochemical device further comprises an internalelectrode extending from the electrochemical element, a tab extendingfrom the internal electrode, and an external electrode extending fromthe tab, the heat-sensitive protective component preferably lies at thelocation where any of the internal electrode, the tab and the externalelectrode is disposed.

[0032] Preferably the electrochemical device of the second embodimenthas the pressure relief means of the first embodiment.

[0033] In a third embodiment, the invention provides an electrochemicaldevice comprising

[0034] a flexible envelope,

[0035] an electrochemical element received and sealed in the envelope,the electrochemical element including internal electrodes and externalelectrodes electrically connected to the internal electrodes andextending outside the envelope, and

[0036] current shut-off means for shutting off either of the electricalconnections between the internal electrodes and the external electrodesby detecting the stress created by inflation of the envelope.

[0037] Preferably the current shut-off means breaks the mechanicalconnection between the internal electrode and the external electrode byutilizing the stress. Preferably the internal electrode and the externalelectrode are attached to the envelope such that the connection betweenthe internal electrode and the external electrode is preferentiallybroken by the stress. Provided that the internal electrode is attachedto the envelope at a tensile strength A, the external electrode isattached to the envelope at a tensile strength B, and the internalelectrode is connected to the external electrode at a tensile strengthC, C is lower than A, and C is lower than B.

[0038] Preferably the electrochemical device of the third embodiment hasthe pressure relief means of the first embodiment and/or the heatsensitive protective component of the second embodiment.

[0039] In all the embodiments, the electrochemical device typicallycomprises a lithium secondary battery.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIGS. 1A and 1B illustrate a sheet-shaped battery according to afirst embodiment of the invention, FIG. 1A being a plan view of anenvelope prior to insertion of an electrochemical element, FIG. 1B beinga plan view of the envelope having the electrochemical element containedtherein.

[0041]FIGS. 2A and 2B illustrate a sheet-shaped battery according toanother embodiment of the invention, FIG. 2A being a perspective view ofan extended envelope prior to insertion of an electrochemical element,FIG. 2B being a perspective view of the envelope having theelectrochemical element contained therein.

[0042]FIG. 3 is a graph showing a relief pressure versus a resincomposition.

[0043]FIG. 4 is a schematic side view of an electrochemical deviceaccording to a second embodiment of the invention.

[0044]FIG. 5 is a plan view of the device of FIG. 4.

[0045]FIG. 6 is a plan view of the electrochemical device used inExample B-1.

[0046]FIG. 7 is a plan view of the electrochemical device used inExample B-2.

[0047]FIG. 8 is a schematic cross-sectional view of an electrochemicaldevice according to a third embodiment of the invention.

[0048]FIG. 9 is a view similar to FIG. 8, showing the envelope in theinflated state.

[0049]FIG. 10 is an exploded perspective view showing the structure ofthe electrochemical device of FIG. 8.

[0050]FIG. 11 is a perspective view of an electrochemical deviceaccording to a further embodiment of the invention.

[0051]FIG. 12 is a schematic cross-sectional view of an electrochemicaldevice according to a still further embodiment of the invention.

[0052]FIG. 13 is a view similar to FIG. 12, showing the envelope in theinflated state.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] Embodiment 1

[0054] In a first embodiment of the invention, an electrochemical deviceincludes an envelope having a sealable opening and an electrochemicalelement having terminals. The electrochemical element is sealed in theenvelope. The envelope is formed of a sheet having a resin layer on itsinner side disposed adjacent to the electrochemical element. Once theelectrochemical element is inserted into the envelope through theopening, the opening is sealed. According to the invention, a strip of amaterial different from the resin layer of the envelope sheet isdisposed in at least a portion of the seal so that the strip serves as ameans for relieving pressure within the envelope. More particularly, theopening of the envelope sheet is sealed by joining the opposed edgeportions of the resin layer on the envelope sheet to form a seal. Informing the seal, the strip is interposed between the resin layer edgeportions, which are joined together whereby the strip forms the pressurerelief means or venting seal. The strip is preferably made of a resinmixture of a first resin adhesive to the resin layer of the envelopesheet, which is most preferably the same resin as the resin layer of theenvelope sheet, and a second resin non-adhesive to the resin layer. Theresin layer of the envelope is preferably comprised of polyolefin resinssuch as polypropylene and polyethylene. Since polypropylene andpolyethylene are not adhesive to each other, a combination ofpolypropylene and polyethylene is recommended.

[0055] The term “non-adhesive” means that when an attempt is made tojoin a sheet of a first resin and a sheet of a second resin together bysuitable means such as heat, they are not joined or bonded because nochemical admixing occurs at their interface. The term “adhesive” meansthat two resin sheets can be joined and bonded together as a result ofchemical admixing at their interface.

[0056] The strip intervenes in the seal of the envelope such that thestrip defines a region of weak bonding force in the seal, which servesas a pressure relieving means or venting seal. When gases generate toincrease the internal pressure of the envelope, the pressure reliefmeans allows the gases to be discharged. The electrochemical device isendowed with the function of an explosion-proof valve and thus improvedin safety and reliability. The bonding force of the strip is usuallyabout 50 to 90% of the bonding force of the remaining seal when thepressure contemplated herein is of an order as in Examples to bedescribed later.

[0057] In a preferred embodiment, the pressure relief means can beformed by interposing the strip of resin mixture between the resin layeredge portions of the envelope and joining them as by thermal fusion. Theseal forming operation is simple. Since the strip is positioned withinthe seal, it causes no disadvantage from the energy density standpoint.Normally, the strip has some bonding force and the same width as theremaining seal and eliminates the problem of moisture ingress from theoutside. The relief pressure of the valve can be adjusted by changingthe mixing ratio of different resins in the strip. The desired reliefpressure can be set simply by furnishing a strip made of a mixture ofdifferent resins in a prescribed ratio. The seal forming operation issimple in this regard too. More particularly, the relief pressure of thevalve increases as the proportion of the first resin adhesive to theresin layer of the envelope increases, and inversely, the reliefpressure decreases as the proportion of the adhesive resin decreases.Most often, the proportion of the adhesive resin is less than theproportion of the non-adhesive resin.

[0058] In a preferred embodiment, the resin mixture of which the stripis made contains the first resin adhesive to the resin layer of theenvelope (desirably the same polyolefin resin as the resin layer) andthe second resin non-adhesive to the resin layer of the envelope(desirably another polyolefin resin) in a weight ratio of from 40/60 to15/85.

[0059] Preferably the strip of the resin mixture is prepared byintimately mixing the first resin adhesive to the resin layer of theenvelope and the second resin non-adhesive to the resin layer of theenvelope and sheeting the mixture to a thickness of 10 to 200 microns,preferably 50 to 150 microns. This sheeting may be done by knownmethods.

[0060] The shape of the strip is not critical. The strip may havevarious shapes including rectangular (inclusive of square), circular,elliptic and triangular shapes although rectangular strips are oftenused. One side of a rectangular strip is preferably equal to the widthof the seal. This is simply achieved by cutting the strip and theenvelope together. The other side of the rectangular strip preferablyhas a distance of about 2 to 20 mm, more preferably about 4 to 15 mm,but is not limited thereto because the relief pressure largely variesdepending on seal-forming conditions.

[0061] The position of the strip in the seal is not critical. Usuallythe terminals extend through the seal of the envelope, and the strip ispreferably disposed in the portion of the seal excluding the location ofthe terminals. For a particular shape of the envelope, the seal portionis bent or otherwise worked. In this case, since the relief pressure ofthe valve is changed by bending, the strip is preferably disposed in aportion of the seal outside the bend. Also preferably, the strip ispositioned nearer to a substantially central axis of the electrochemicalelement in order that the strip serve as the pressure relief valve,although the position of the strip also depends on the shape of theenvelope.

[0062] Referring to FIGS. 1A and 1B, there is illustrated a sheet-shapedelectrochemical device according to the first embodiment of theinvention. FIG. 1A is a plan view of a bag-like envelope. FIG. 1B is aplan view of electrochemical device comprising a electrochemical elementreceived in the envelope.

[0063] As shown in FIG. 1A, the envelope 20 is a bag obtained by foldinga sheet of laminate film and joining opposite sides by thermal fusion toform a pair of first seal portions 21. In the plan view of FIG. 1A, thebag envelope 20 is rectangular and has the fold at the lower side and anopening at the upper side. The fold and the seal portions 21 define aninterior space.

[0064] As shown in FIG. 1B, an electrochemical element 10 havingterminals 13, 14 is inserted-into the interior space of the envelope 20through the opening such that the terminals 13, 14 extend outwardthrough the opening. The opening-defining opposed portions of theenvelope 20 between which the terminals 13, 14 are interposed are joinedby thermal fusion to form a second seal portion 22, completing anelectrochemical device 1A. The device 1A is structured such that theelectrochemical element 10 is sealed within the envelope 20 and theterminals 13, 14 extend outward through the second seal portion 22.

[0065] The electrochemical element 10 includes positive and negativeelectrodes and a solid polymer electrolyte (SPE). Each electrodeincludes a current collector such as a aluminum foil or copper foil towhich a coating material of an active material, a binder, etc. isapplied. To the positive and negative electrodes are connected theterminals 13, 14. The terminals 13, 14 include sealed regions 13 a, 14 awhere they are covered with the second seal portion 22.

[0066] The bag-like envelope 20 is formed by folding a sheet because theextra area which is otherwise necessary for sealing of the lower end iseliminated. In this embodiment, a strip 23 is preferably disposed in thesecond seal portion 22 to provide a valve function. More preferably, thestrip 23 is disposed in a portion of the second seal portion 22 outsidethe location of the terminals 13, 14, that is, outside the sealedregions 13 a, 14 a. In the illustrated embodiment, the strip 23 isdisposed substantially at the center between the terminals 13 and 14.The position of the strip 23 is not limited to the center, and may beanywhere as long as an area enough to accommodate the strip isavailable.

[0067] It is noted that the electrochemical element 10 with terminals13, 14 is subjected to the prescribed treatments including immersion inan electrolytic solution before it is contained in the envelope 20.While the terminals 13, 14 are extended outside and the strip 23 isinterposed between the opening-defining edge portions of the envelope20, the opening-defining edge portions of the envelope are joined byheat compression or thermal fusion to form the second seal portion 22.

[0068] The electrochemical device is not limited to the embodiment ofFIGS. 1A and 1B and may be constructed as shown in FIGS. 2A and 2B.FIGS. 2A and 2B illustrate an electrochemical device using an envelopehaving a recess substantially corresponding to the size of theelectrochemical element. FIG. 2A is a perspective view showing anextended envelope and FIG. 2B is a perspective view of theelectrochemical device.

[0069] As shown in FIG. 2A, a rectangular envelope sheet 30 has a recess31 capable of accommodating the electrochemical element 10 which is thesame as in FIGS. 1A and 1B except for terminals 13, 14, not shown inFIG. 2A. In the recess 31, the electrochemical element 10 is placedfollowing the prescribed treatments (including immersion in electrolyticsolution). A strip 34 is placed on a seal-forming portion 33 of theenvelope sheet 30 surrounding the recess 31. The envelope sheet 30 isfolded along a line 32. The mated portions of the envelope sheet arejoined by heat compression or thermal fusion to form a seal 33 (see FIG.2B).

[0070] Constructed in this way is an electrochemical device 18 as shownin FIG. 2B. The electrochemical element 10 is accommodated and sealed inthe envelope 30 while terminals 13, 14 extend out of the envelope 30.The strip 34 serving as a valve is positioned in the seal 33 and near toa substantially central axis of the electrochemical element 10 andsandwiched between the opposed portions of the envelope 30.

[0071] The electrochemical device of the invention is not limited to theillustrated embodiments, and modifications are made thereto. Theenvelope may be configured to any desired shape depending on aparticular purpose or application.

[0072] The heat compression or thermal fusion to form a seal can beeffected by known methods.

[0073] The envelope used in the electrochemical device of the inventionmay be formed of a sheet material which is relatively unbreakable andbondable, which does not undergo chemical changes upon contact with theelectrochemical element, and which prevents leakage of electrolyticsolution and gas permeation. A typical sheet is a laminate film in theform of a metal layer such as aluminum which is coated on oppositesurfaces with a thermal fusible resin layer such as a polyolefin resinlayer (e.g., polypropylene or polyethylene) and a heat resistantpolyester resin layer. The thermal fusible resin layer such as apolyolefin resin layer becomes the resin layer on the inner side to bein contact with the electrochemical element.

[0074] The thermal fusible resin layer on the inner side of the laminatefilm is often a single layer although it may have a multilayerstructure. The thickness of the resin layer (total thickness in the caseof two or more layers) is preferably about 30 to 130 microns. Thisthickness range ensures to form an effective seal on thermal fusion. Ifthe resin layer is thinner below the range, it may become difficult toform an effective seal around the terminals because the terminals aregenerally about 50 to 100 microns thick. If the resin layer is thickerbeyond the range, heat may not be fully transferred from a heating plateto the terminals, failing to form an effective seal. The thick resinlayer adds to the overall thickness of the battery against the demandfor size reduction.

[0075] The metal layer is often a single layer although it may have amultilayer structure. The thickness of the metal layer (total thicknessin the case of two or more layers) is preferably about 15 to 150microns.

[0076] The heat resistant resin layer on the outer side of the metallayer preferably has a thickness of about 10 to 50 microns (totalthickness in the case of two or more layers). The laminate filmpreferably has an overall thickness of about 50 to 200 microns. A heatresistant resin layer may be additionally formed between the metal layerand the thermal fusible resin layer in order to ensure insulation. Whenprovided to this end, the additional heat resistant resin layerpreferably has a thickness of about 5 to 20 microns.

[0077] The thermal fusible resins used herein are typically polyolefinresins including polypropylene and polyethylene (including high densitypolyethylene, low density polyethylene, linear low density polyethyleneand polyethylene base ionomers). Also useful are acid-modifiedpolyolefin resins, for example, polyethylene modified with acid such ascarboxylic acid, and acid-modified polypropylene obtained by graftpolymerization of maleic anhydride. These acid-modified polyolefinresins are advantageously used herein because carboxyl groups thereoncontribute to improved adhesion.

[0078] The acid-modified polypropylene obtained by graft polymerizationof maleic anhydride is commercially available under the trade name ofAdmer from Mitsui Chemicals Co., Ltd. Among the Admers, polypropylenetype Admers are preferred. Especially preferred are homopolymers AdmerQF305 (melting point 160° C.) and QF500 (melting point 165° C.) andethylene copolymers Admer QF551 (melting point 135° C.), QB540 (meltingpoint 150° C.), QB550 (melting point 140° C.), and QE060 (melting point139° C.). Similar resins are commercially available under the trade nameof Modic from Mitsubishi Chemical Corp. The Modic polypropylene resinsinclude a homopolymer Modic P502, and random copolymers Modic P513V,P505 and P517.

[0079] The heat resistant resins are preferably polyester resins such aspolyethylene terephthalate (PET) and polyamide resins.

[0080] The metal layer of aluminum etc. may be a metal foil or anevaporated film.

[0081] The terminals may be foils of various metals and alloys such asaluminum, nickel, copper and stainless steel which may be surface coatedwith titanium, tantalum, chromium, zinc, nickel or tin. The terminalsoften take the form of a lead of rectangular or circular cross section.Terminals of aluminum and nickel are preferred. To enhance the adhesionto the envelope, adhesives such as the acid-modified polyolefin resinsdescribed above may be applied to the seal regions of the terminals tocome in contact with the envelope.

[0082] Embodiment 2

[0083] In a second embodiment of the invention, the electrochemicaldevice is illustrated in FIG. 4 as comprising an envelope 102, anelectrochemical element 103 received in the envelope 102, and aheat-sensitive protective component 105 for protecting theelectrochemical element 103. The heat-sensitive protective component 105is disposed on the envelope 102 substantially outside theelectrochemical element 103.

[0084] In a preferred embodiment, provided that the electrochemicaldevice has a maximum thickness D₁ where the electrochemical element isreceived and a maximum thickness D₂ where the heat-sensitive protectivecomponent is disposed, D₁ is equal to or greater than D₂ (i.e., D₁≧D₂).In a preferred embodiment, the heat-sensitive protective component isdisposed at the location where any of an internal electrode, a tab andan external electrode is disposed.

[0085] Since the heat-sensitive protective component 105 is disposed onthe envelope 102 substantially outside the electrochemical element 103,the heat-sensitive protective component 105 can be attached withoutincreasing the maximum thickness of the electrochemical device.

[0086] As shown in FIG. 4, the electrochemical device 101 includes theenvelope 102, the electrochemical element 103 received in the envelope102, an internal electrode extending from the electrochemical element, atab 104 extending from the internal electrode, and an external electrode106 extending from the tab 104 and out of the envelope 102. (In theillustrated embodiment, the tab is integral with the internalelectrode.) In the region of the envelope 102 which is substantiallyoutside the electrochemical element 103, the envelope 102 has sealedtherein only the internal electrode, tab 104 or external electrode 106.Therefore, the region of the envelope 102 outside the element 103 is aslim region having a reduced thickness (or height). The location of theheat-sensitive protective component 105 in the slim region (having themaximum thickness D₂) provides effective utilization of an extra spaceand does not increase the maximum thickness D₁ of the electrochemicaldevice 101.

[0087] The heat-sensitive protective component 105 may be attachedeither outside or inside the envelope 102. For ease of attachment, it isrecommended that the component 105 be attached outside the envelope 102.For reliable operation and protection from the ambient atmosphere, thecomponent 105 may be attached inside the envelope 102.

[0088] Since the maximum thickness D₃ of the heat-sensitive protectivecomponent 105 is usually smaller than the difference between the maximumthickness D₁ of the electrochemical device and the maximum thickness D₂of the slim region (i.e., D₁−D₂≧D₃), the location of the heat-sensitiveprotective component 105 in the slim region does not increase themaximum thickness D₁ of the electrochemical device 101. The maximumthickness D₁ of the electrochemical device 101 is not less than themaximum thickness D₂ of the slim region where the heat-sensitiveprotective component 105 is disposed (i.e., D₁≧D₂).

[0089] In the preferred embodiment, the heat-sensitive protectivecomponent 105 is disposed at the location where any of the internalelectrode, the tab 104 and the external electrode 106 is disposed asbest shown in FIG. 5. In the case of a lithium secondary battery, theelectrochemical device 101 has the electrochemical element 103 sealed inthe envelope 102, the electrochemical element 103 being a laminate of apositive electrode (internal electrode), an electrolyte and a negativeelectrode (internal electrode). The external electrodes or out-leads 106from the electrochemical element extend out of the envelope 102. Theopen ends (left side in FIGS. 4 and 5) of the envelope 102 with theexternal electrodes 106 sandwiched are joined by thermal fusion to forma seal portion. Since the internal electrodes, tabs and externalelectrodes are usually made of metals having relatively enhanced heattransfer, they serve to transfer heat from the interior. As aconsequence, the heat-sensitive protective component located on theregion of the envelope where such metal members are sealed is improvedin operating sensitivity. Differently stated, the heat-sensitiveprotective component can be operated at a temperature closer to theinternal temperature.

[0090] In the region of the envelope which is substantially outside theelectrochemical element, a temperature difference of 10° C. or more issometimes found between the zone where the internal electrodes, tabs orexternal electrodes are located and the other zone where they are not.

[0091] The arrangement that the heat-sensitive protective component 105is disposed at the location where any of the internal electrode, the tab104 and the external electrode 106 is disposed means that the overlapbetween the projected area of the heat-sensitive protective componentand the projected area of the internal electrode, tab or externalelectrode, as viewed from above in FIG. 5, is at least 50%, morepreferably at least 70%, and most preferably at least 80% of theprojected area of the heat-sensitive protective component.

[0092] The heat-sensitive protective component 105 used herein is notcritical as long as it can detect the heat release of theelectrochemical element and shut off or restrict current flow at apredetermined temperature and has a sufficient shape to dispose in theabove-specified position.

[0093] Exemplary heat-sensitive protective components are thermal fusesand PTC elements. Organic PTC elements or thermistors are based on theconstruction that electrically conductive particles are dispersed in aresin matrix as disclosed, for example, in JP-B 64-3322 and JP-B4-28743. By adjusting the type and proportion of the resin, conductiveparticles and other components, the organic PTC element can be readilycontrolled in operating temperature and be endowed with so-calledhysteresis characteristics. Owing to resin molding, the organic PTCelement can be freely designed to an optimum shape to fit in thespecified position.

[0094] The heat-sensitive protective component may be secured to theenvelope using a heat conductive paste or sheet, which is preferred fromthe heat transfer standpoint. The securing may be done by various meansincluding adhesives, adhesive tapes, screws, rivets and pressure bondingof an outer casing. Use of heat conductive adhesives is preferred. Theadhesive for bonding the protective component should preferably one thatdoes not experience a weakening of the bonding force in the temperaturerange between −40° C. and 160° C. Typically, heat conductive epoxyadhesives are useful.

[0095] The heat-sensitive protective component may be attached on eitherthe positive electrode side or the negative electrode side or both aslong as it can shut off or restrict the current flow.

[0096] Embodiment 3

[0097] A third embodiment of the invention provides an electrochemicaldevice comprising a flexible envelope and an electrochemical elementreceived and sealed in the envelope. The electrochemical elementincludes internal electrodes and external electrodes electricallyconnected to the internal electrodes and extending outside the envelope.The device further comprises current shut-off means for shutting offeither of the electrical connections between the internal electrodes andthe external electrodes by detecting the stress created by inflation ofthe envelope.

[0098] In a preferred embodiment, the current shut-off means breaks themechanical connection between the internal electrode and the externalelectrode by utilizing the stress. The internal electrode and theexternal electrode are attached to the envelope such that the connectionis preferentially broken by the stress. More specifically, the internalelectrode is attached to the envelope at a tensile strength A and theexternal electrode is attached to the envelope at a tensile strength B,and the internal electrode is connected to the external electrode at atensile strength C such that C is lower than A, and C is lower than B(i.e., A>C and B>C). It is preferred that A>2C and B>2C, especially A>5Cand B>5C.

[0099] Conventional adhesives are used in attaching the internalelectrode and the external electrode to the envelope at tensilestrengths A and B, respectively. More particularly, adhesives for use insealing the external electrode in the seal portion of the envelope aresuitable. Exemplary adhesives are carboxylic acid and similaracid-modified polyethylene, acid-modified polypropylene, epoxy resinsand modified isocyanate adhesives.

[0100] The tensile strengths at which the internal and externalelectrodes are attached to the envelope are not critical insofar as theabove-described relationship is met. The tensile strength per unit areais often in the range of about 20 to 100 gf/mm², especially about 30 to65 gf/mm².

[0101] Means for attaching the internal electrode to the externalelectrode at tensile strength C may be conventional means for joiningmetal members such as adhesive bonding, welding, soldering and brazing.Among these, ultrasonic welding is preferred because the bond stress isreadily adjustable. In the ultrasonic welding, the bond stress isadjusted by controlling any one of the ultrasonic power, welding timeand applied pressure or two or more such factors in combination.

[0102] The tensile strength at which the internal electrode is attachedto the external electrodes is not critical insofar as theabove-described relationship is met. The tensile strength per unit areais often in the range of about 3 to 30 gf/mm², especially about 6 to 13gf/mm².

[0103] When the envelope is inflated, a stress of about 20 to 100 gf/mm²is usually exerted.

[0104] Referring to FIG. 8, there is illustrated an electrochemicaldevice according to the third embodiment of the invention. FIG. 9 is aview similar to FIG. 8, showing the state that the envelope 202 isinflated by gas release.

[0105] The electrochemical device of FIG. 8 includes an electrochemicalelement having a stack of positive electrode layers 204, electrolytelayers 205 and negative electrode layers 206, which is sealed in anenvelope 202. The electrochemical element is received in the envelope202 such that an external electrode or out-lead 203 extend out of theenvelope 202. Open ends of the envelope 202 with the external electrode203 sandwiched therebetween are joined by thermal fusion to form a sealportion 207. The electrochemical device is configured such that theelectrochemical element is sealed within the envelope 202 and theexternal electrode 203 extends outward through the seal portion 207. Atthis point, the interior of the envelope 202 is substantially ventedwhereby the envelope comes in close contact with the electrochemicalelement. It is noted that FIGS. 8 and 9 show a portion of theelectrochemical element and envelope which is disposed near the externalelectrode.

[0106] In the illustrated embodiment, the external electrode 203 issecured to the top side of the envelope 202 by an adhesive layer 208while the (lowermost) internal or positive electrode 204 is secured tothe bottom side of the envelope 202 by an adhesive layer 209. Theexternal electrode 203 is connected to the internal or positiveelectrode 204 by a joint 211.

[0107] On an abnormal event such as over-charging, gases are generatedto inflate the envelope 202. Specifically, the envelope 202 is dilatedupward and downward directions as viewed in FIG. 8 so that a stress actsto separate the external and internal electrodes 203 and 204 at thejoint 211. On further inflation by gas generation, the stress byinflation eventually surpasses the bond stress C between the externalelectrode 203 and the internal or positive electrode 204, breaking thebond or connection of the joint 211. That is, the connection between theexternal and internal electrodes 203 and 204 is broken. Breakage ofconnection may occur on the positive electrode side or the negativeelectrode side or both. Once the connection on either side is broken,current flows no longer, insuring safety.

[0108] At this point, since the stresses A and B at which the externaland internal electrodes 203 and 204 are secured to the envelope 202 bythe adhesive layers 208 and 209, respectively, are greater than the bondstress C between the external and internal electrodes 203 and 204,adhesive failure at the joint 211 preferentially occurs. Since adhesivefailure suddenly occurs after the envelope 202 is inflated to asubstantial extent, the external and internal electrodes 203 and 204 areinstantaneously separated at the same time as failure as shown in FIG.9. In this way electrical connection is shut off. Since the inflatedstate is maintained thereafter, the connection is not restored.

[0109] The electrochemical element is illustrated in FIG. 10 as amultilayer structure comprising positive and negative electrodes 204 and206 each in the form of a metal foil such as aluminum or copper foil,and solid polymer electrolyte layers 205, which are alternately stacked.To the positive and negative electrodes 204 and 206 are connectedexternal electrodes or out-leads 203. It is noted that the externalelectrode to the negative electrodes is omitted in FIGS. 8 and 9. Theexternal electrodes each are formed of a metal foil of aluminum, copper,nickel, stainless steel or the like. The external electrode 203 includesa sealed region covered with the adhesive layer 207.

[0110] The electrochemical element used herein is not limited to thesecondary battery of the multilayer structure shown in FIGS. 8 and 9,and secondary batteries of the wrap structure shown in FIGS. 11 to 13and capacitors of similar structure may be equivalently used.

[0111] Referring to FIG. 11, a secondary battery 221 is constructed bywinding a sandwich of electrolyte 224 between negative and positiveelectrodes 223 and 225. A tack tape 227 is attached at the end of thepositive electrode 225 to maintain the structure wrapped. The negativeand positive electrodes 223 and 225 have external electrodes (orout-leads) 222 and 226, respectively. It is seen from FIG. 12 that theexternal electrode 226 is connected to the internal electrode 225 by arelatively weak joint 231.

[0112] The secondary battery 221 is sealed in an envelope 230 as shownin FIG. 12. The secondary battery 221 including internal electrode(positive electrode) 225 is secured to the bottom of the envelope 230with an adhesive 229. The external electrode 226 is secured to the topof the envelope 230 with an adhesive 228.

[0113] If the envelope 230 is inflated by any abnormal operation, thejoint 231 between the external electrode 226 and the internal electrode225 is broken so that the electrodes are suddenly separated as shown inFIG. 13.

[0114] The electrochemical device of the invention becomes advantageouswhen any of Embodiments 1 to 3 is applied. If desired, two or more ofEmbodiments 1 to 3 are combined. These combinations are arbitraryalthough the combination of all three embodiments ensures the bestsafety feature to the electrochemical device. The heat-sensitiveprotective component may be omitted if an electric equipment in whichthe electrochemical device is to be mounted is furnished with aprotective circuit or if the circuitry associated therewith does notrequire.

[0115] The electrochemical element used in the electrochemical device ofthe invention is typically a lithium secondary battery, but not limitedthereto. A capacitor of a similar structure may also be used.

[0116] The electrochemical device of the invention can be typicallyembodied as lithium secondary batteries and electric double-layercapacitors, which are described below.

[0117] Lithium Secondary Battery

[0118] The structure of a lithium secondary battery according to theinvention is not critical, although it is generally constructed by apositive electrode, a negative electrode and an electrolyte and appliedto lamination batteries and rectangular batteries.

[0119] The electrodes to be combined with the electrolyte are selectedfrom well-known electrodes for lithium secondary batteries and arepreferably formed of a composition comprising an electrode activematerial, a gel electrolyte and optionally, a conductive aid.

[0120] The negative electrode is composed of a negative electrode activematerial such as carbonaceous materials, lithium metal, lithium alloy oroxide materials while the positive electrode is composed of a positiveelectrode active material such as oxide or carbonaceous materialscapable of intercalating and deintercalating lithium ions. Using suchelectrodes, a lithium secondary cell having satisfactory properties isobtainable.

[0121] The carbonaceous material used as the active material may beproperly selected from mesocarbon microbeads (MCMB), natural orartificial graphite, resin fired carbon materials, carbon black, andcarbon fibers. They are used in powder form. Preferred of these isgraphite desirably having a mean particle size of 1 to 30 μm, especially5 to 25 μm. Too small a mean particle size tends to lead to a shortenedcharge/discharge cycle life and an increased variation of capacity. Toolarge a mean particle size may lead to a remarkably increased variationof capacity and a reduced average capacity. Such a large mean particlesize causes a variation of capacity because the contact between graphiteand the current collector and the contact between graphite particlesbecome inconsistent.

[0122] Compound oxides containing lithium are preferred as the oxidecapable of intercalating and deintercalating lithium ions. Exemplaryoxides are LiCoO₂, LiMn₂O₄, LiNiO₂, and LiV₂O₄. Powders of these oxidespreferably have a mean particle size of about 1 to 40 μm.

[0123] Conductive aids are added to the electrode composition ifdesired. Preferred conductive aids include graphite, carbon black,carbon fibers, and metals such as nickel, aluminum, copper and silver.Graphite and carbon black are especially preferred.

[0124] Preferably the electrode composition contains the activematerial:conductive aid:gel electrolyte in a weight ratio of30-90:3-10:10-70 for the positive electrode, and in a weight ratio of30-90:0-10:10-70 for the negative electrode. The gel electrolyte usedherein is not critical and selected from conventional ones. Electrodesfree of the gel electrolyte are also acceptable. In this case, a bindersuch as fluoro-resin and fluoro-rubber is used in an amount of about 3to 30% by weight.

[0125] Electrodes are prepared by first dispersing the active materialand optional conductive aid in a gel electrolyte solution or bindersolution to form a coating solution. The coating solution is thenapplied to a current collector. The coating means is not critical andmay be properly determined in accordance with the material and shape ofthe current collector. In general, metal mask printing, electrostaticdeposition, dip coating, spray coating, roll coating, doctor blade,gravure coating, and screen printing techniques are used. Thereafter,rolling is carried out by a platen press or calender roll if necessary.

[0126] The current collector may be properly selected from conventionalcurrent collectors in accordance with the shape of a device in which thebattery is used and the manner of disposing the current collector in acasing. In general, aluminum and analogues are used for the positiveelectrode and copper, nickel and analogues are used for the negativeelectrode. The current collector is usually in the form of metal foil ormetal mesh. Although the metal mesh has a lower contact resistance withthe electrode than the metal foil, even the metal foil provides asufficiently low contact resistance.

[0127] The solvent is then evaporated off, completing the electrode. Thecoating thickness is preferably about 50 to 400 μm.

[0128] The polymeric membrane used as the electrolyte is typicallyselected from polymeric microporous membranes of polyethylene oxide(PEO), polyacrylonitrile (PAN) and polyvinylidene fluoride (PVDF).

[0129] Also, separators may be used as disclosed in JP-A 9-219184, JP-A2000-223107 and JP-A 2000-100408.

[0130] The positive electrode, polymeric membrane and negative electrodeare stacked in this order and compressed, providing an electrochemicalelement.

[0131] The electrolytic solution with which the polymeric membrane isimpregnated is generally composed of an electrolyte salt and a solvent.The electrolyte salts used herein include lithium salts such as LiBF₄.LiPF₆, LiAsF₆, LiSO₃CF₃, LiClO₄, and LiN(SO₂CF₃)₂.

[0132] [0046]

[0133] The solvent used in the electrolytic solution is not critical aslong as it is compatible with the solid polymer electrolyte andelectrolyte salt. For lithium batteries, polar organic solvents which donot decompose even at a high operating voltage are useful, for example,carbonates such as ethylene carbonate (abbreviated as EC), propylenecarbonate (PC), butylene carbonate, dimethyl carbonate (DMC), diethylcarbonate and ethyl methyl carbonate, cyclic ethers such astetrahydrofuran (THF) and 2-methyltetrahydrofuran, cyclic ethers such as1,3-dioxolan and 4-methyldioxolan, lactones such as γ-butyrolactone, andsulfolane. Also useful are 3-methylsulfolane, dimethoxyethane,diethoxyethane, ethoxymethoxyethane, and ethyl diglyme.

[0134] In the electrolytic solution consisting of the solvent and theelectrolyte salt, an appropriate concentration of the electrolyte saltis about 0.3 to 5 mol/l. The highest ion conduction is achieved at about1 mol/l.

[0135] When the polymeric membrane is immersed in the electrolyticsolution, the membrane absorbs the electrolytic solution and gels,becoming a solid polymer electrolyte.

[0136] When the composition of solid polymer electrolyte is representedby copolymer/electrolytic solution, a proportion of the electrolyticsolution in a range of about 40 to 90% by weight is preferred formembrane strength and ion conduction.

[0137] Electric Double-layer Capacitor

[0138] Although the structure of the electric double-layer capacitor isnot critical, usually an electrolyte is interleaved between a pair ofpolarizing electrodes and an insulating gasket is disposed at theperiphery of the polarizing electrodes and the gel electrolyte. Such anelectric double-layer capacitor may be any of the paper and laminatetypes.

[0139] The polarizing electrodes are prepared by adding a binder such asfluoro-resin or fluoro-rubber to a conductive active material such asactivated carbon or activated carbon fibers, and shaping the mixtureinto sheets serving as the electrodes. The content of the binder isabout 5 to 15% by weight. A gel electrolyte may also be used as thebinder.

[0140] A current collector made of platinum or conductive rubber such asconductive butyl rubber may be used in the polarizing electrodes. Thecurrent collector may also be formed by thermal spraying of metals suchas aluminum and nickel. Metal mesh may be attached to one side of theelectrode layer.

[0141] In the electric double-layer capacitor, the polarizing electrodesas described above are combined with a solid polymer electrolyte.

[0142] The polymeric membrane used as the electrolyte is typicallyselected from polymeric microporous membranes of polyethylene oxide(PEO), polyacrylonitrile (PAN) and polyvinylidene fluoride (PVDF).

[0143] The electrolyte salts used herein include (C₂H₅)₄NBF₄,(C₂H₅)₃CH₃NBF₄ and (C₂H₅)₄PBF₄.

[0144] Well-known non-aqueous solvents may be used in the electrolyticsolution. Preferred are electrochemically stable non-aqueous solventssuch as propylene carbonate, ethylene carbonate, 7-butyrolactone,acetonitrile, dimethylformamide, 1,2-dimethoxyethane and sulfolane aloneor mixtures of any.

[0145] In the electrolytic solution consisting of the nonaqueous solventand the electrolyte, an appropriate concentration of the electrolyte isabout 0.1 to 3 mol/l.

[0146] When the polymeric membrane is immersed in the electrolyticsolution, the membrane absorbs the electrolytic solution and gels,becoming a solid polymer electrolyte.

[0147] When the composition of solid polymer electrolyte is representedby copolymer/electrolytic solution, the preferred proportion of theelectrolytic solution is about 40 to 90% by weight.

[0148] An insulator such as polypropylene and butyl rubber may be usedas the insulating gasket.

[0149] The envelope is as described in Embodiment 1 although theenvelopes of Embodiments 2 and 3 are also useful.

[0150] For example, the envelope is formed from a laminate film in theform of a metal layer such as aluminum which is coated on oppositesurfaces with a thermal fusible resin layer such as a polyolefin resinlayer (e.g., polypropylene or polyethylene) and a heat resistantpolyester resin layer. A pair of laminate films are overlapped andjoined along three sides by thermal fusion of the thermal fusible resinlayers to form a first seal portion, obtaining a bag having one openside. Alternatively, one laminate film is folded and two opposite sidesare joined by thermal fusion to form first seal portions, obtaining abag having one open side.

[0151] The laminate film is preferably of a multilayer structure ofthermal fusible resin layer/polyester resin layer/metal foil/polyesterresin layer from the inside to the outside in order to establishinsulation between the metal foil in the laminate film and the outputterminals of the electrochemical element. Since the use of such alaminate film permits the high-melting point polyester resin layer to beleft intact during the thermal fusion, the spacing between the terminalsand the metal foil of the envelope is maintained, ensuring insulation.To this end, the polyester resin layer of the laminate film shouldpreferably have a thickness of about 5 to 100 microns.

EXAMPLE

[0152] Examples of the invention are given below by way of illustrationand not by way of limitation.

Example A-1

[0153] Polypropylene (PP) and polyethylene (PE) were mixed in apredetermined ratio, milled at 190° C., and sheeted into a sheet ofabout 100 μm thick. The sheet was slit into strips of 5 mm wide.

[0154] As the envelope, a laminate film of PET (12 μm)/Al (20 μm)/PET(12 μm)/PP (80 μm) was folded and heat sealed, into a bag as shown inFIG. 1A. The PP layer was inside the bag. The first seal portions 21were 8 mm wide.

[0155] Without insertion of an electrochemical element, a strip 23 wasplaced between laminate film edges and approximately at the center ofthe second seal portion 22. The strip 23 had a size of 5 mm on one side(the strip's own width). During the bag forming process, the strip wascut together with the open edges so that the other side of the strip hadthe same size (4 mm) as the second seal portion 22. The bag had overalldimensions of 56 mm×59 mm.

[0156] An air inlet port was formed in the major surface of the heatsealed envelope. Air was pumped into the envelope interior through theinlet port. The operating or relief pressure at which the strip 23opened as a valve was measured. FIG. 3 is a graph showing a reliefpressure (Pa) versus a resin composition, PP/(PE+PP) in % by weight.

[0157] It is seen from FIG. 3 that the relief pressure increases as theproportion in the strip-forming resin mixture of PP (which is the sameas the innermost PP layer of the laminate film) increases. In order thatthe strip function as a pressure relief valve, an appropriate proportionof PP in the resin mixture is in a range of about 15 to 40% by weight.

Example A-2

[0158] An electrochemical device as shown in FIG. 1B was fabricated.

[0159] Positive electrodes were formed by applying a slurry of LiCoO₂,carbon black, graphite and PVDF in N-methylpyrrolidone (NMP) solventonto an aluminum foil. For the convenience of battery construction, thepositive electrodes were of two types, one having a coating on onesurface of a 100-μm foil and one having coatings on both surfaces of a20-μm foil. Negative electrodes were formed by applying a slurry ofMCMB, carbon black, PVDF and NMP onto both surfaces of a copper foil of10 μm thick. The separator used was a porous membrane of PVDF having athickness of 40 μm.

[0160] The electrodes and separator were cut to a predetermined shape.These sheet sections were stacked in the order of positiveelectrode/separator/negative electrode/separator/positive electrode . .. while applying a spot of adhesive to each sheet section at its centerand fusing the adhesive at 110° C. The adhesive used was anethylene/methacrylic acid copolymer. There was obtained an adhesivelytacked laminate. Terminals were formed by welding aluminum and nickelribbons to the positive and negative electrode tabs, respectively. Thelaminate was immersed in a solution containing 1 mol of LiPF₆ per litterof a mixture of EC and DMC in a volume ratio of 1:2. After gelation, theexcessive electrolytic solution was removed. The amount of theelectrolytic solution impregnated was 2.8 to 3.0 g.

[0161] The envelope sheet was a laminate film of PET (12 μm)/Al (20μm)/PET (12 μm)/PP (80 μm) as used in Example 1, which was processedinto a bag as shown in FIG. 1A. The PP layer was on the inside of thebag to come adjacent to the electrochemical element.

[0162] The laminate was inserted into the aluminum laminate bag throughthe open edge portion, a strip 23 was placed in the edge portion 22 asshown in FIG. 1B, and the edges were heat fused to form the seal portion22. The entire assembly was heat pressed at 80° C. and 29.4×10⁴ Pa (3kgf/cm²) for 1 minute for integrating the structure within the laminatebag.

[0163] The strip used was formed of a mixture of PP and PE in a weightratio of 1/4 and of the same size as in Example 1.

[0164] The thus fabricated battery is designated sample No. 1. A batterydesignated sample No. 2 was fabricated by the same procedure as sampleNo. 1 except that the strip was omitted.

[0165] An over-charging test with a current flow of 1 ampere waseffected on battery sample Nos. 1 and 2. For the normal seal battery(sample No. 2) without an explosion-proof valve, the envelope failed,sometimes with a fire hazard, before a charge quantity of 250% wasreached. For the battery with an explosion-proof valve (sample No. 1),the valve smoothly opened when the envelope was inflated at a chargingquantity of 200%, avoiding failure and ignition.

[0166] Battery sample No. 1 was aged at 60° C. and RH 90% for 20 days,finding no degradation of performance by moisture ingress (such asbulging of the battery).

Example A-3

[0167] An electrochemical device as shown in FIG. 2B was fabricated.

[0168] By using an electrochemical element and a laminate film as inExample A-2, placing the electrochemical element in the recess 31 of theenvelope sheet 30 as shown in FIG. 2A, placing the strip 34 in theseal-forming portion 33, and folding the envelope sheet at the line 32,a battery was fabricated. The battery had overall dimensions of 62 mm×42mm (excluding projecting terminals). The seal portion 33 was 4 mm wideon the side where the terminals extended and 4 mm wide on the othersides. The strip 34 was made of the same material as in Example 1 andhad dimensions of 4 mm×5 mm, with one side being equal to the width ofthe seal portion.

[0169] The thus fabricated battery is designated sample No. 3. A batterydesignated sample No. 4 was fabricated by the same procedure as sampleNo. 3 except that the strip was omitted.

[0170] An over-charging test was effected on battery sample Nos. 3 and 4as in Example A-2. The results were the same as in Example A-2corresponding to the presence or absence of the strip. Sample No. 3 wassubjected to an aging test as in Example A-2, finding no degradation ofperformance by moisture ingress.

Example A-4

[0171] Four battery samples were fabricated as in Examples A-2 and A-3except that polyethylene (PE) was used instead of PP on the inside ofthe laminate film and the strip was made of a resin mixture of PE and PPin a weight ratio of 1/4. These samples were examined by the same testsas in Examples A-2 and A-3. The results were the same as in Examples A-2and A-3 corresponding to the presence or absence of the strip.

Example B-1

[0172] Using a polyimide base adhesive tape, a PTC thermistor 105 of 0.5mm thick having a resistance of 8 mΩ at room temperature and a nominaloperating temperature of 75° C. was attached to a battery having amaximum thickness of 4.0 mm at the position shown in FIG. 6. After theattachment of the PTC thermistor, the battery had a maximum thickness of4.0 mm. No heat conductive paste was applied between the thermistor andthe envelope. Using a polyimide base adhesive tape, a K typethermocouple 108 was attached to the battery at the position shown inFIG. 6 for measuring the temperature of the battery surface. The batterywas charged up to a terminal voltage of 8.0 V by conducting a constantcurrent of 2.2 A. Thereafter, charging was continued while the currentwas controlled so as to maintain the voltage of 8.0 V.

[0173] After about 16 minutes from the start of the test, the batterysurface reached a temperature of about 66° C. at which the PTCthermistor operated. Due to the increased resistance of the PTCthermistor, the terminal voltage increased from 4.7 V to 8 V andthereafter, the current flow declined. Even after 2 hours from the startof charging, the current flow did not increase beyond 200 mA. Neitherfailure nor ignition occurred.

Example B-2

[0174] The charging test was carried out as in Example B-1 except thatthe PTC thermistor 105 was placed at the position shown in FIG. 7.

[0175] After about 20 minutes from the start of the test, the batterysurface reached a temperature of about 82° C. at which the PTCthermistor operated. Due to the increased resistance of the PTCthermistor, the terminal voltage increased from 4.7 V to 8 V andthereafter, the current flow declined. Even after 2 hours from the startof charging, the current flow did not increase beyond 200 mA. Neitherfailure nor ignition occurred. However, the temperature rose beyondabout 90° C. at maximum, so that the envelope was inflated by the gasgenerated.

[0176] Since the batteries used in these examples were relatively smallsized, the attachment position of the PCT thermistor had littleinfluence. For large size batteries for use in personal computers, theattachment position of the PCT thermistor will have more influence.

Comparative Example B

[0177] The charging test was carried out as in Example B-1 except thatthe PTC thermistor was omitted.

[0178] The terminal voltage increased to 4.67 V at maximum. After about29 minutes from the start of the test, failure and ignition occurred.

[0179] It is noted that although the PTC thermistor was used as theheat-sensitive protective component in Examples B-1, B-2 and ComparativeExample B, substantially equivalent results were obtained on use of athermal fuse.

Example C

[0180] A battery of the construction shown in FIG. 8 was fabricated byalternately stacking negative electrodes, gel electrolyte layers andpositive electrodes. An aluminum foil lead 203 serving as the externalelectrode was welded to the tap of the positive electrode collector 204serving as the internal electrode, by means of a ultrasonic welder. Anickel foil was welded to the tap of the negative electrode collector206, by means of an electric resistance welder. The joint (211) betweenthe positive electrode 204 tab and the lead 203 had a tensile strengthof 63 gf/mm².

[0181] This was inserted into an envelope 202 made of an aluminumlaminate film. The lead 203 was joined to the laminate film 202 withmaleic acid-modified polypropylene (208). The joint (208) between thelead and the laminate film had a tensile strength of 63 gf/mm². Theopening of the laminate film was sealed in vacuum, completing thebattery.

[0182] The battery was charged up to a terminal voltage of 5.0 V byconducting a constant current of 1.0 ampere. Thereafter, charging wascontinued while the current was controlled so as to maintain the voltageof 5.0 V. After about 66 minutes from the start of the test, theenvelope started to inflate due to gas generation. About 30 secondslater, the joint between the positive electrode collector tab and thelead was broken whereby the current flow was shut off.

[0183] The current blocked state was then kept, inhibiting failure andignition.

Comparative Example C

[0184] The charging test was carried out as in Example C except thatneither the positive electrode collector tab nor the lead was adhesivelyjoined to the laminate film.

[0185] After about 65 minutes from the start of the test, the envelopestarted to inflate due to gas generation. The current continued,provoking failure and ignition after 70 minutes from the start of thetest.

[0186] Although the explosion-proof mechanism, protective component andcurrent shut-off mechanism are independently assessed in the aboveExamples, the electrochemical device can be made more safe by combiningtwo or more of them. In particular, better results were obtained from acombination of Example A with Example C.

[0187] There has been described a reliable and safe electrochemicaldevice having a simple venting means for venting gas in response to arise of the internal pressure, the venting means serving as anexplosion-proof valve and preventing ingress of moisture andcontaminants. There has also been described an electrochemical devicecomprising an envelope of flexible film having a protective componentattached thereto, which is designed, without changing the maximumthickness of the device, to ensure that heat is transferred from theinterior to the protective component whereby the device has improvedsafety. There has also been described an electrochemical device,typically secondary battery, comprising an envelope of flexible film anda failsafe mechanism capable of preventing current flow on a gasgenerating accident.

[0188] Japanese Patent Application Nos. 2000-75988, 2000-181676 and2000-181677 are incorporated herein by references.

What is claimed is:
 1. An electrochemical device comprising an envelopehaving a sealable opening, an electrochemical element having terminals,said electrochemical element being inserted in said envelope through theopening which is sealed to form a seal, said envelope having a resinlayer on its inner side adjacent said electrochemical element, and astrip of a material different from the resin layer of said envelope,disposed in at least a portion of the seal, said strip serving as ameans for relieving pressure within said envelope.
 2. Theelectrochemical device of claim 1 wherein the sealable opening of saidenvelope is defined by opposed portions of the resin layer of theenvelope, said strip is interposed at least in part between the opposedportions of the resin layer, and a seal is formed by joining the opposedportions of the resin layer together with said strip to provide thepressure relief means.
 3. The electrochemical device of claim 1 whereinsaid strip is made of a resin mixture of a first resin adhesive to theresin layer of the envelope and a second resin non-adhesive to the resinlayer.
 4. The electrochemical device of claim 1 wherein the resin layerof the envelope is made of a first polyolefin resin, and said strip isdisposed in contact with the resin layer and made of a resin mixture ofthe first polyolefin resin and a second polyolefin resin non-adhesive tothe first polyolefin resin.
 5. The electrochemical device of claim 4wherein either one of the first and second polyolefin resins comprisespolypropylene, and the other resin comprises polyethylene.
 6. Theelectrochemical device of claim 4 wherein the resin mixture of whichsaid strip is made contains a more amount of the second polyolefin resinthan the first polyolefin resin.
 7. The electrochemical device of claim6 wherein the resin mixture of which said strip is made contains thefirst polyolefin resin and the second polyolefin resin in a weight ratioof from 40/60 to 15/85.
 8. The electrochemical device of claim 1 whereinthe terminals extend through the seal of said envelope, and said stripis disposed in the portion of the seal excluding the location of theterminals.
 9. An electrochemical device comprising an envelope, anelectrochemical element received and sealed in the envelope, and aheat-sensitive protective component for protecting the electrochemicalelement, said heat-sensitive protective component being attached to saidenvelope substantially outside said electrochemical element.
 10. Theelectrochemical device of claim 9 wherein the electrochemical device hasa maximum thickness D₁ where said electrochemical element is receivedand a maximum thickness D₂ where said heat-sensitive protectivecomponent lies, D₁ being equal to or greater than D₂.
 11. Theelectrochemical device of claim 9 further comprising an internalelectrode extending from said electrochemical element, a tab extendingfrom the internal electrode, and an external electrode extending fromthe tab, wherein said heat-sensitive protective component lies at thelocation where any of the internal electrode, the tab and the externalelectrode is disposed.
 12. The electrochemical device of claim 9 whichcomprises a lithium secondary battery.
 13. The electrochemical device ofclaim 9 which has the pressure relief means of claim 1 .
 14. Anelectrochemical device comprising a flexible envelope, anelectrochemical element received and sealed in said envelope, saidelectrochemical element including internal electrodes and externalelectrodes electrically connected to the internal electrodes andextending outside said envelope, and current shut-off means for shuttingoff either of the electrical connections between the internal electrodesand the external electrodes by detecting the stress created by inflationof said envelope.
 15. The electrochemical device of claim 14 whereinsaid current shut-off means breaks the mechanical connection between theinternal electrode and the external electrode by utilizing the stress.16. The electrochemical device of claim 14 wherein the internalelectrode and the external electrode are attached to the envelope suchthat the connection between the internal electrode and the externalelectrode is preferentially broken by the stress.
 17. Theelectrochemical device of claim 14 wherein the internal electrode isattached to the envelope at a tensile strength A, the external electrodeis attached to the envelope at a tensile strength B, and the internalelectrode is connected to the external electrode at a tensile strength Csuch that C is lower than A, and C is lower than B.
 18. Theelectrochemical device of claim 14 which comprises a lithium secondarybattery.
 19. The electrochemical device of claim 14 which has thepressure relief means of claim 1 .
 20. The electrochemical device ofclaim 14 which has the heat-sensitive protective component of claim 9 .21. The electrochemical device of claim 20 which further has thepressure relief means of claim 1 .